This invention relates to steam turbine machines and more particularly, to a bucket cover design for turbine buckets or blades with inclined tips, and to a labyrinth seal arrangement for interaction with the modified bucket cover.
Bucket covers are typically connected to the radially outer tips of turbine buckets or blades to dampen vibration and to prevent twisting of the blades. The bucket covers also cooperate with seal devices such as labyrinth seal teeth for minimizing leakage of gas or steam past the bucket tips. Typically, bucket covers comprise small, flat, uniform-thickness plates attached to the tips of the buckets by reception of a tenon on the bucket tip within an opening in the cover, with subsequent deformation of the tip of the tenon to secure the cover in place. For certain turbine stages, in the low pressure section of a steam turbine for example, the bucket tip is inclined relative to horizontal. A uniform thickness cover on the inclined bucket tip, however, reduces the surface area available for interaction with the generally horizontally oriented labyrinth teeth because of axial movement of the rotor relative to the stator partitions of turbine labyrinth teeth, and thus results in increased tip leakage losses. For certain turbine stages, this design requires that the bucket cover itself be provided with additional seal teeth but this arrangement still does not sufficiently reduce tip leakage losses, nor does it promote commonality of seal configurations among different turbine stages.